How to Calculate Battery Reserve Capacity

Look on the label on the electrical device that your battery powers to find out the milliamp hour consumption. Write it down on a sheet of paper using a pen. You need this figure to calculate battery reserve capacity. For example, it may say 400 mAh.

Look on the side of the battery to see what mAh capacity the battery produces when it's new and fully charged. Write down the figure on the same sheet of paper. For example, a nickel metal hydride battery is likely to produce about 800 mAh when fully charged. Using the example of 400 mAh and 800 mAh, if you divide 400 into 800, you get two. This means that a fully charged battery will power your device for two hours.

Turn on the multimeter and set the dial to measure mAh. Put the metal sensor that's on the end of the red cable attached to the multimeter onto the positive battery terminal. Put the sensor on the end of the black cable from the meter onto the negative terminal. Read the meter display and write down the figure. For example, it may say 200 mAh.

Calculate the battery reserve capacity. Divide the mAh reading from the meter by the mAh reading you got from the battery label. For example, if the meter read 200 mAh and the battery capacity is 800, then 200 divided by 800 is 25. This means your battery has 25 percent reserve capacity left.

Calculate how long the battery reserve capacity will power your electrical device. Divide the meter reading by the mAh you wrote down from the electrical device's label, then multiply by 60. The result is the number of minutes you can expect your battery to power the device. For example, if the meter reads 200 mAh and the electrical device consumes 400 mAh, 200 divided by 400 equals 0.5. Multiply 0.5 by 60 to get 30. Using this example, the battery reserve capacity will power your device for 30 minutes.